warr 1996 standardised clay crystall data low grade nz

8/7/2019 Warr 1996 standardised clay Crystall data low grade NZ

1/14

Eur. J. Mineral.1996,8, 115-127

Standardized clay mineral crystallinity data from the verylow-grade metamorphic facies rocks of southern New ZealandL A U R E N C E N . WARR

Geologisch-Palontologisches Insti tut , Ruprecht-Karls-Universitt , Im Neuenheimer Feld 234,D-69120 Heidelberg, Germany

Abstract: X-ray diffraction determined clay mineral crystallinity data, standardized to the Crystallinity IndexStandard (CIS) scale, is presented for illite (white mica) and chlorite-bearing pelitic rocks of the zeolite,prehnite-pumpellyite, pum pellyite-actinolite and the greenschist facies. Go od correlations exist between bothillite and chlorite crystallinity and crystallite size data, with respect to the mineral (metabasite) facies conditions, using both the Full-Width-at-Half-Maximum and a Siemens Single-Line Fourier method, whilecalculations employing the Warren-Averbach method yielded unacceptably high degrees of error, and hencepoor correlations. These preliminary data conform with the commonly held view that the diagenetic-, anchi-,and epi-zones of Kiibler's illite crystallinity boun dary lim its, adopted by the C IS, correspond w ith the zeolite,prehnite-pumpellyite plus pump ellyite-actinolite, and greenschist facies, respectively. The C IS scale is therefore considered a reliable indicator of metamorphic grade, when adequate attention is given to the calibrationof the experimental data and the selection of suitable peak reflections. The air-dried Sr- or Ca-saturated illite002 and chlorite 003 peak com bination are recomm ended as two useful reflections for accurate and rapid XR Dgrade determinations in very low-grade metamorphic rocks.Key-words: Crystallinity Index Standard (CIS) scale, very low-grade metamorphic facies, southern NewZealand.

Introduct ionIn the field of very low-grade metamorphism, alarge num ber of s tudies have com pared the degreeof X-ray determined illite (white mica) crystallinity (IC) with other indicators of metamorphicconditions, such as mineral facies data, coal rank,fluid inclusion data, conodont coloration, and iso-topic information, from a range of similar andcontrasting geological settings (see review ofKisch, 1987).Traditionally, correlations with the mineral(metabasite) facies divisions of the lower gradesof metamorphism have played an important rolein the interpretation of IC data; the definition ofKiibler 's (1967) anchizone/epizone boundary wasoriginally based on the occurrence of greenschistfacies minerals. Subsequently, a wide range ofsuch comparisons have been drawn from numerous areas, which, according to Kisch (1987), con

form to a general pattern with the diagenetic zonecorresponding to the zeolite facies, the anchizoneto the prehnite-pumpellyite and pumpellyite-actinolite facies, and the epizone either to thepumpellyite-actinolite or greenschist facies. Anumber of apparent discrepancies to this scenariohave also been reported, such as epizone valuesoccurring with prehnite-pumpellyite facies assemblages (Robinson & Bevins, 1986; Bevins & Robinson, 1993), anchizone values with zeolitefacies assemblages (Durney, 1974; Aprahamian etal., 1975) and even diagenetic values reportedwith prehnite-pumpellyite facies assemblages(Bril & Thiry, 1976; Kiibler et al., 1979; Arkai ,1983; Robinson & Bevins, 1986).Such inconsistencies in the data are usuallytaken to be geologically meaningful and a rangeof explanations have been put forward. Kisch(1987) suggested three possible reasons, namely,the non-diagnostic nature of the mineral facies

0935-1221/96/0008-0115 $ 3.25) 1996 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart


2/14

116 L. N. Warrassemblages, the inhibition of clay mineral growthby induration during a prior metamorphism, andthe influence of rock strain enhancing IC. Furthermore, Arkai (1991) proposed varying amounts ofdetrital mica s in the pelites as a principal cause forsuch discrepancies.Differences between IC data and mineralfacies assemblages may also be anticipated byconsidering the reaction mechanisms involved.The mineral facies concept is one based on thelaws of thermodynamics, in which mineral reactions are reversible and equilibrium assemblagesare attained within given P-T fields, ideally independe nt of the P-T-t path involved. A s a result, pe-trogenetic grids based either on experimentalstudies of mineral stabilities (Liou et al, 1985) orinternally-consistent thermodynamic data, havebeen successfully used for establishing P-T conditions from mineral assemblages in metamorphicrocks (Frey et al, 1991). On the other hand, theIC concept is based on continuous, structural andchemical transformations which occur within theprograde illite-muscovite series (Hunziker et al,1986). These kinetically controlled and irreversible transformations are considered to takeplace through a series of metastable phase transitions governed by Ostwald ripening processes(Eberl et al, 1990). The IC method, hence, provides a measure of reaction progress which is pre-dictively more dependent on the time function ofthe P-T-t path than the thermodynamically controlled minerals facies. A universal correlation between these two metamorphic indicators wouldonly be expected if P-T-t paths in very low-grademetamorphic rocks are everywhere relatively consistent, which is clearly not the case (Robinson,1987).The main restriction in drawing purposefulcorrelations between IC studies, and which applyto clay mineral crystallinity investigations in general, is that the numerical data produced by laboratories are frequently incompatible, due to insufficient standardization practices (Kisch & Frey,1987; Warr & Rice, 1994; Krumm et al, 1994;Krumm et al, in press) . Although many laboratories have compared differences due to variationsin machine conditions, using polished rock chipstandards (Kisch, 1990), inconsistencies arisingfrom sample preparation effects have not beensufficiently calibrated (Krumm et al, in press).Similar problems are encountered when comparing clay mineral crystallite size data betweenresearch groups, which is further complicated by

the wide range of measurement methods, saturating cations and peak reflections used for analysis.Crystallite sizes have been calculated by theScherrer equation, based on the air-dried illite 001peak (Weber et al, 1976; Kiibler, 1984; Merrimanet al, 1990; rodori & Elsass, 1994), the Wilsonmethod (Wilson, 1963), on the air-dried illite 001(Arkai & Toth, 1983) or illite 002 reflection (Nie-to & Sanchez-Navas, 1994), and the Warren-Aver-bach (W-A) method (Warren & Averbach, 1950),on the glycolated Sr-saturated illite 002-005 combination (Eberl & rodori, 1988; Eberl et al,1990; Warr & Rice, 1994), or the Ca-saturated air-dried illite 002 peak (Eberl & Blum, 1993). As theac- curacy and precision of these methods are stillpoorly tested, comparisons against other quantitative methods of analysis are critical in order to establish their suitability as indicators of the conditions of metamorphism.In order to facilitate a single compatible database and thus enable more effective comparisonsbetween clay mineral crystallinity and crystallite(domain) size studies, Warr (1993) and Warr &Rice (1994) recently introduced a calibration approach to the standardization of data using a set ofavailable rock chip standards. This allows datasets to be directly and quantitatively compared,despite differences in analytical procedures adopted. A standardized scale of measurement was introduced, called the Crystallinity Index Standard(CIS) , which was equated as closely as possible tothe original scale of Kiibler (1967) by calibration,using the polished interlaboratory standards ofKisch (1990).The purpose of this paper is to present a ran geof standardized clay mineral crystallinity andcrystallite size data for illites and chlorites, basedon the analysis of 14 pelitic samples selected fromthe well studied very low-grade metamorphicfacies of southern New Zealand. The applicabilityof various XRD methods, as indicators of the conditions of low temperature metamorphism, are explored, and some preliminary P-T constraints forthe CIS scale, presented.

Geology and sample descript ionThe South Island of New Zealand consists of a series of accreted litho-tectonic terranes (Coombs etal , 1976) which were extensively deformed andmetamorphosed during the Jurassic-CretaceousRangitata Orogeny (Fleming, 1970). Continuousand widespread transitions in metamorphic grade


3/14

C lay mineral crystallinity and very low-grade metam orphism 117and deformational intensity are well exposedwithin the Torlesse (zeolite to amphibolite facies),Caples (zeolite to greenschist facies), Murihiku(zeolite facies), and Brook Street (greenschist facies) terranes, making it a classical area for bothlow temperature metamorphic and structural studies. The largest and most extensively studied ofthe terranes is that of the Torlesse, consistingmostly of Permian to early Cretaceous interbed-ded greywacke and argillite lithologies, whichgrade into schist toward the south (Otago Schist)and the west (Alpine Schist). The greywacke-suitewas derived from a dominantly plutonic-metamor-phic provenance with a lesser volcanic and sedimentary component (MacKinnon, 1983), deposited along the Pacific Margin of Gondwana. Welldefined metamorphic and textural zones havebeen mapped across the Torlesse transition intoOtago Schist (Bishop, 1972; Norris & Bishop,1990). Although isograd and isotect boundariesare often locally oblique, a general correlation between metamorphism and deformation has beenrecognized in greywacke sandstone lithologies.

Undeformed rocks of textural zone 1 are typicallyof zeolite or prehnite-pumpellyite facies, texturalzone 2 (weakly to well cleaved) are commonly ofthe prehnite-pumpellyite or pumpellyite-actinolitefacies, and textural zones 3 and 4 (phyllitic toschistose) are usually of the greenschist facies.Peak metamorphism was characteristically one ofa high pressure facies series as indicated by thewhite mica b0 values of Sassi & Scolari (1974),with temperatures of 400-450 C (Yardley, 1982)and pressures ca . 4.5kbar (Jamieson & Craw,1987).These well exposed very low-grade metamorphic terranes provide an ideal study area forcorrelating mineral facies assemblages against claymineral crystallinity data. Firstly, the relativelyporous and permeable greywacke lithologies wereparticularly susceptible to regional metamorphism, with abundant intermediate and mafic igneous fragments favoring the development of hydrousCa-Al silicate assemblages. Secondly, the grey-wackes are frequently interbedded with finegrained clay-rich pelites and tuffs suitable for claymineral crystallinity studies. Thirdly, there was

Table 1. Summary of the 14 pelitic samples used in the study, showing locations, the stop numbers listed in Coombs& C ox (199 1), textural zones, reported m ineral (metabasite) facies, minerals identified in the < 2 m fraction by XR D,and illite intensity ratio (Ir) data by the method of rodori (1984). Numbers greater than 1 indicate the presence ofinterlayered smectite.Sample

NZ29N Z8N Z 4N Z9NZ11N Z 4 1N Z 4 2N Z 4 5NZ23NZ15-PNZ15-SN Z 4 6NZ17N Z 4 3P = phyll

Locality

Rocky Point Quarry, MossburnBenmore Dam (Shore platform)Deep Creek (Waimate Gorge)Dalrachnie Bridge (Longslip Creek)Longslip CreekWatsons BeachTaieri MouthLake Hawea to Lake Wanaka roadsectionRemarkablesLindis River (south of Goodger R oad)Lindis R iver (south of Goodger R oad)Lake Hawea to Lake Wanaka roadsectionShotover River BridgeLake Hawea to Lake Wanaka roadsection

Stop

41.3C1.11.41.55.45.5-2.61.71.7-2.1-

ite, S = schist, # = non-diagnostic mineral facies

Texturalzone1112A2B223A3333B44

Mineral facies

ZeoliteZeolitePrh-pmpPrh-pmpPrh-ActPmp-bearing#Pmp-bearing#

GreenschistGreenschistGreenschistGreenschistGreenschistGreenschistGreenschist

< 2 m assemblage

IU/Mus, Chi, Kin, Ab, Kfs, Qtz,Lmt, SmIll/Mus, Chi, Ab, Kfs, QtzIll/Mus, C hi, QtzIll/M us, C hi, Ab, Kfs, Qtz, SmIll/Mus, Chi, Ab, Kfs, QtzIll/Mus, C hi, Kin, A b, SmIll/Mus, Chi, Kin, Ab, Kfs, Sm,Stp, Pm pIll/Mus, Chi, Ab, Kfs, SmIll/Mus, C hi, Ab, KfsIll/Mus, C hi, A b, KfsIll/Mus, Chi, Ab, Kfs, StpIll/Mus, C hi, A b, KfsIll/Mus, C hi, A b, KfsIll/Mus, C hi, Ab, Kfs

Ir

1.51.21.51.01.01.01.01.01.01.01.01.01.11.0


4/14

11 8 L. N. Wan-only minor igneous activity during metamorph-ism, so the effects of local contact metamorphism,which may complicate correlations, can be avoided.

Locality and mineralogical information forthe 14 pelitic samples used in this study, comprising mudstones, phyllites and schists selectedfrom across the region, are summarized in Table1. Full details of the collection sites are given inthe field guide of Coombs & Cox (1991), fromwhich most of the following summary is drawn.The samples cover the range of zeolite to green-schist facies metamorphism, and span all of thefour textural zones defined by Bishop (1972).Two uncleaved (textural zone 1) hard siltymudstones of the zeolite facies were collected, thefirst (NZ29) from Rocky Point Quarry, Mossburn,and the second from the shore platform of theBenmore Dam (NZ8). Laumontite-bearing assemblages have been described from the Rocky PointQuarry, and were detected in the < 2 m fractionof sample NZ29, along with traces of discretesmectite. Also recorded from this locality is avitrinite reflectance value of 0.94 % R m oil(Kisch, 1981), indicating a maximum temperatureof about 150C (using the equation from Barker& G oldstein, 1990). A t the Benmore Dam locality,metamorphic conditions were near the zeolite toprehnite-pumpellyite facies transition, as laumon-tite is partly replaced along small fractures byprehnite and quartz. Five pelitic samples from textural zones 1 and 2 consist of two weakly cleavedsilty mudstones (N Z4 and N Z 9) both of prehnite-pumpellyite facies, a phyllite (NZ11) from a pum-pellyite-actinolite facies locality, and two tuf-faceous samples (NZ41 and NZ42), from localities with non-diagnostic pumpellyite-bearingassemblages. The remaining seven samples comefrom textural zones 3 and 4, and consist of green-schist facies grade phyllites (NZ45, NZ15-P,NZ46 , NZ43) and sch is t s (NZ23 , NZ17 andNZ15-S) .In order to make a preliminary assessment ofthe expandable nature of the illites, the intensityratio method of rodori(1984) was applied (Table1) , which showed small amounts of il l i te/smectitemixed-layering to be present in only three samples(Ir values between 1.2-1.5). All other pelitesyielded Ir values close to 1, indicative of non-expandable illites.

Analyt ical methods and data cal ibrat ionSamples were prepared and analyzed followingthe methods outlined in Warr & Rice (1994),us ing th ick (>3mg/cm 2 ), sedimented, Sr-saturat-ed clay fractions, with measurements made usinga Siem ens D 5000 diffractometer at 40 kV and 30mA and Cu K radiat ion, aperture diaphragms al l1, detector diaphragm 0.05, graphite monochro-mator and a time constant of 1 s. Three XRD slidepreparations from each sample were scanned bothin the air-dried and glycolated conditions from 2to 50, at a scan-rate of 0.629/min and a step-width of 0.01. All data presented are mea n v aluesbased on measurements of three slide preparationsper sample (n = 3).

Baosal illite 001 ( -10-) , 002 (5-) , 003(3.3-) and 005 (2-) reflections and chlorite001 ( -14-) , 002 (*7-) , 003 ( -4 .7-) , 004(-3.5-) reflections were fitted using the programFIT (Siemens version 3.0) by the following treatment of the raw data f ile. A linear backgroundlevel was removed and selected reflections fittedusing a Split Pearson VII function. Various peak-fitting trials were performed in order to establishthe most reliable peak-fitting requirements, whichsatisfactorily modelled both the peak profile andbackground level. Although K 2 was retained forpeak-breath measurements at low 20 angles (tomaintain better counting statistics), the influenceof this reflection was subtracted by peak fittingprior to crystallite size calculations. Suitably fittedprofiles were transferrred to the WIN-CRYSIZE(version 1.0) program and calculations made fromsingle peaks and peak-pair combinations, usingboth the Warren-Averbach (W-A) method and aSiemens Single-Line (S-L) Fourier method.Further details about these methods are given inthe Siemens WIN-CRYSIZE (1991) handbookand in Eberl & Blum (1993).The W-A method (Warren & Averbach, 1950)is considered to be the more ideal method forcalculating the crystallite size of clay minerals, asit can yield a crystallite size distribution from themodelled diffraction profile (Eberl & rodori,1988; Eberl & Blum, 1993). In summary, it separates the crystallite size coefficient (A L*), which isindepende nt of the order of reflection, and the distortion (microstrain) coefficient (A L D), which is afunction of the order of reflection, from the Fourier coefficients (A n) of at least two basal reflections (Warren & Averbach, 1950; Warren, 1959;Klug & Alexander, 1974). The method can also be


5/14

Clay mineral crystallinity and very low-grade metam orphism 119

used on single reflections, if the broadening effectis assumed to arise from one effect only (e.g. crystallite size). The Single-Line (S-L) method (Siemens W IN-CRY SIZE, 1991) also uses the FourierTransform of the pure broadening function, but itassumes that small crystallites cause peak broadening in the form of a Lorentzian curve, and thatstrain causes peak broadening in the form of aGaussian curve (Warren-Averbach, 1950). Although the S-L method works with only one peakreflection, and is intended from peaks which arebroadened both by crystallite size and microstraineffects, calculations were notably more preciseand faster (over six times quicker) than the W-Amethod.

All crystallite size calculations were madeusing the machine broadening profiles determinedon the single-crystal mica standard (MF1C) ofWarr & Rice (1994) for which the basal reflectionFWHM data is available. As no suitable single-crystal chlorite flake could be found, and attemptsto produce sufficiently narrow reflections usingcoarsely crystal l i te chlori te-bearing samplesproved unsuccessful, a "chlorite" standard was assembled by modification of a MF1 C fitted datafile. The nearest neighbor basal mica reflectionwas used, to reduce any 20 dep endant differences,and the peak positions were adjusted to correspond with the chlorite reflections of a naturalsample (NZ45), displaying rationally spaced reflections with the minimal of peak interference,the highest peak intensities and the narrowestpeak-widths. This standard is considered realistic,as the machine broadening profile should be similar for both muscovite and chlorite, regardless ofthe difference in d-spacing. It also discounts anydiscrepancies that may be introduced by using different mineral standards, which can be variablyinfluenced by factors other than the machinebroadening.

Experimental data was converted to calibrated CIS data using the procedure and standards ofWarr & Rice (1994). Clay mineral crystallinity,measured as the Full-Width-at-Half-Maximum(FWHM), was calibrated using the regression datashown in Fig. 1. The excellent linear fit of experimental to calibrated values yielded a R 2 value of0.996, based both on air-dried and glycolatedpreparations. Crystallite (domain) size (L) data didnot require any conversion as no difference wasdetected between experimental runs and the published standardized L values for the CIS samples,when using compatible measurement methods.

0.8 j 26

-o -4 t f I *[.Q I f% 0.2 I > *==> I s

0 I r 1 1 i 10 0.2 0.4 0.6 0.8

Calibrated (CIS) data (FWHM) 2BFig. 1. Data used to convert experimental FWHM datato calibrated Crystallinity Index Standard data: CIS data= (uncalibrated data/1.0377) + 0.04634, R2 = 0.996.

ResultsDiffraction characteristics and measurementprecisionClay mineral crystallinity and crystallite size datafor the basal reflections of illite and chlorite areshown in Table 2. Overall, the FWHM data fromglycolated (GY) slides are similar to air-driedpreparations (within the limits of error), exceptfrom the following differences. The three leastmetamorphosed samples (NZ29, NZ8 and NZ4),show slight reductions in illite 001 peak-widths( 1 0 - 1 4 % narrower) due to the presence of smallquantities of interlayered smectite. All other samplesshowed no significant narrowing of this peak,confirming the absence of mixed-layered smectite, as indicated by the preliminary expandabilitydata presented in Table 1. Samples containingsmall amounts of discrete smectite (NZ29, NZ9,NZ41 and NZ45), show significant narrowing ofthe chlorite 001 peak-width upon glyeolation, asthe 001 smectite reflection shifts from around62 0 in the air-dried state to about 5.2 26 w henglycolated. The presence of sm ectite in these samples is most likely due to the effects of secondarylow temperature hydrothermal alteration or recentweathering.

During glycolation of three greenschist faciessamples (NZ23, NZ15-P and NZ46), significantincreases in peak-breadths were incurred at lowtwo theta angles, affecting both illite and chloritereflections. This uncommon feature is most prob-


6/14

120 L. N. Wan-Table 2. Clay mineral crystallinity and crystallite size data for the 14 pelitic samples from southern New Zealand.Symbols used: # denotes two non-diagnostic pumpellyite-bearing samples; FWHM = Full-width-at-half-maximum;W-A = Warren-Averbach method; S-L = Single-Line method; GY = Glycolated. All FWHM values are given in unitsof 2 , in contrast to W-A and S-L crystallite size thickness (L), which are expressed in nm. * marks peak reflectionsin which Kcc2 radiation was retained during analysis.

Mineral Fades Zeolite Prh-Pmp and Pmp-Act GreenschistNZ29 NZ8 NZ 4 NZ9 NZ11 NZ41# NZ42# NZ45 NZ23 NZ15-P NZ15-S NZ46 NZ17 NZ43 % error

n=30lllite001 FWHM*002 FWHM*003 FWHM*005 FWHM

0.62 0.51 0.280.50 0.43 0.280.56 0.46 0.220.76 0.70 0.34

0.320.300.280.33

0.270.250.210.19

0.310.310.330.32

0.23 0.18 0.170.20 0.17 0.160.21 0.17 0.170.19 0.14 0.15

0.190.180.180.13

0.250.200.220.19

0.190.180.180.14

0.230.200.200.14

0.250.200.200.15

61075001 FW HM * G Y002 FW HM * G Y003 FW HM * G Y0 05 F W H M G Y

001 W- A002 W-A003 W-A005 W-A

001 S-L002 S-L003 S-L005 S-L

001-002 W- A001-003 W -A001-005 W- A002-003 W -A002-005 W-A003-005 W -AAll peaks W- A

0.560.580.380.80

828321113162712

797253215415

0.460.510.380.68

72224913171812

77723243911

0.240.250.230.33

3938602328324428

31393933429242

0.310.310.330.32

2124392325272227

31393933429242

0.260.250.200.18

2248582333363358

19212440515639

0.300.320.300.33

2644353227272728

24262750453635

0.230.210.180.17

42361464637539271

434042243619050

0.180.180.180.14

31741101306086133170

262931596910346

0.250.210.200.17

195966645573118125

16181954596930

0.210.210.200.14

2458641075572108189

20232554545436

0.240.230.200.18

26461213535407264

232527314714315

0.270.230.210.15

215383895070105151

18202141508132

0.210.200.190.16

18434961385789129

15171839414626

0.220.200.190.15

21525972345280115

18202246495431

776427221621711107

26232328222019

Chlorite001 FWHM*002 FWHM*003 FWHM*004 FWHM*

0.660.440.440.45

001 FWH M*002 FW HM *003 FWH M*0 04 F W H M *

001 W- A002 W-A003 W-A004 W-A

001 S-L002 S-L003 S-L004 S-L

GYGYGYGY

0.590.490.440.44

161983013191818

001-002 W-A001-003 W-A001 -004 W-A002-003 W-A002-004 W-A003-004 W-AAl l peaks W -A

1415142616513

0.420.590.440.440.440.450.530.41191913261617151718201925171015

0.310.310.34

0.300.22

40157

262520

44483511

0.480.350.370.360.370.380.390.37172012211516201720151878410

0.310.300.270.260.290.300.270.262822276826293339292827201915

0.410.340.290.22

0.350.300.2820271019162425221921133530414

0.450.330.240.170.200.320.260.17532433293234393661575621254835

0.37 0.180.18 0.180.16 0.170.19 0.180.210.180.180.183848569354688012035373743423838

0.250.250.220.21182727414757669716171727241919

0.190.200.180.180.220.200.200.20324554814858679530313140403934

0.240.240.240.230.210.240.220.23284240513837434926282954565435

0.190.200.190.180.290.270.240.21243744654756659722232433333127

0.260.220.210.210.250.210.190.18323429473338456131323135311931

0.380.240.230.200.370.220.200.19

231438

364175

3221

10874361326271611111340343523223827


7/14

Clay mineral crystallinity and very low-grade metamorphism 121

ably the result of a significant loss in the degree ofpreferred particle orientation incurred during gly-colation, which could give rise to small amountsof additional broadening as a result of divergingbeam conditions in the incident X-rays. This effect is only resolvable in the more coarsely crystalline samples, where peak-broadening effectsdue to crystallite size are small.The progressive narrowing of the clay mineral peaks with increasing order of reflection wasa characteristic observed in XRD profiles of thehigher-grade metamorphic samples (ICooi


8/14

122 L . N. Warr

Xwoo

ou

l 2 6 0.1 2 9

1 20

I l l i te 001 (FWHM)c) 100 nm

I l l i t e 001 (FWHM)d)

joU

1 26 100 nmI l l i t e 002 (FWHM) Illite 002 (L)

BEpizone

Anchizone

Diagenetic zone

AD*

Greenschist faciesPumpellyite-actinolite faciesPrehnite-pumpellyite facies 1Zeolite faciesNon-diagnostic pumpellyitebearing assemblages

Fig. 2. Useful peak combinations plotted as log-log graphs, a) Illite 001 FWHM vs i l l i te 002 FWHM, with a best fitlinear regression expressed as: y = 0.0354 + 0.774x, R

2= 0.974. b) Illi te 001 FWHM vs chlorite 002 F W H M . Thebest fit curve is logarithmic, expressed as y = 0.5835 + 0.52045*LOG(x) , R2 = 0.886. The l inear regression y =- 0 . 0 1 6 1 + 1.1438x, R2 = 0.791, shown on the graph, is the best fit if the two zeolite facies samples are not considered ,c) Illite 002 F W H M vs chlori te 003 FW HM . y = - 0 . 0 0 5 3 6 + 1.0903x, R2 = 0.830. d) Illite 002 L vs chlorite 003 Ldetermined by the Single-Line m ethod, y = - 0 . 9 9 2 + 0.9056x, R2 = 0.955.


9/14

Clay mineral crystallinity and very low-grade metamorphism 123

for these data, does not show a 1:1 trend, asshown by FWHM data for these peaks, but suggests the chlorites have slightly smaller crystallitesizes than grade-equivalent illites.Employing the ICQOI anchizonal boundarylimits of Kiibler (1967), which are adopted by theCIS scale, the equivalent boundary limits for theother peak reflect ions are presented in Table 3,based on the regression data given in the captionof Fig. 2. As the CIS crystallite size limits of theanchizone presented in Warr & Rice (1994) weredetermined from the less accurate W-A calculat ions on the glycolated i l l i te 002-005 combinat ions, these limits are considered an approximation only.All selected boundary limits show a goodcorrespondence between clay mineral crystallinitydata and mineral facies assemblages (Fig. 2a-d).The two zeolite facies samples correspond withthe diagenetic zone, the prehnite-pumpellyite andpumpellyite-actinolite facies with the anchizone,and the greenschist facies samples with the epi-zone . Within the boundary l imits presented inTable 3, over 90% of the 65 measurements plotted

Table 3. Preliminary correlation of the anchizonalboundary limits for both illite an d chlorite crystallinity(FWHM in 26) an d crystallite size (L in nm), adoptingth e ICooi limits of Kiibler (1967), presented forcomparative purposes only. All other limits presented ar ebased on linear or logarithmic (*) regression of the datashown in F ig. 2 a -d .

Illite 00 1 Illite 00 2 Illite 00 2(FWHM) (FWHM) (L)

Low-grade anchizonal boundary 0.42 0.36 23

High-grade anchizonal boundary 0.25 0.23 52

Chlorite 00 2 Chlorite 00 3 Chlorite(FWHM) (FWHM) 003 (L)

Low-grade anchizonal boundary 0.42 0.36 20

High-grade anchizonal boundary 0.25 0.23 46

Chlorite 00 2(FWHM)*

Low-grade anchizonal boundary 0.39

High-grade anchizonal boundary 0.27

in Fig. 2 a - d correspond with the expected mineral facies conditions. The position of the twonon-diagnostic pumpellyite-bearing samples wouldsuggest that NZ41, which is of middle to upperanchizonal grade, is of pumpellyite-prehnitefacies, and NZ42, which falls around the upperanchizone/epizone boundary, is of pumpellyite-actinolite facies.

Also shown in Fig. 2d, is the relationship between illite and chlorite crystallite size thicknesses and the assigned textural zones (1-4).Overall, larger L is commonly associated with theincreasing intensity of deformation (as recorded inthe greywacke units), with textural zone 1 samples corresponding mostly to the diagenetic grade,textural zone 2, mostly to the anchizone, and textural zones 3 and 4, largely to the epizone. The relationship is, however, not mutually exclusive,with a textural zone 1 sample occurring in the anchizone, and a textural zone 2 sample occurringnear the anchizone/epizone boundary, along sidetextural zone 4 schists.

Discuss ionProcedures for determining clay mineralcrystallinity and crystallite sizeIdeally, the most suitable basal reflections formeasuring clay mineral crystallinity and crystallite size are those which are broadened minimallyby effects other than crystallite size (the X-rayscattering domain size), and can be precisely measured. Therefore, the narrower, sym metrical, higherintensity basal reflections, which show minimalinterference by other mineral phases, and allowprecise fitting of both the peak profile and thebackground level, are preferred. Realistically,however, these criteria are rarely satisfied, and theoptimum reflections will vary with the mineralogyof samples.

The illite 001 air-dried peak has been traditionally used for crystallinity studies, following thework of Kbler (1967, 1968, 1984, 1990). However, while this relatively intense reflection hasproven adequate for the general assignment ofmetamorphic grade, in illites with significantswelling properties, it is strongly broadened(asymmetrically) by the presence of interlayeredsmectite (Moore & Reynolds, 1989; Stern et al.,1991; Nieto & Sanchez-Navas, 1994). Morerecently, Eberl & Blum (1993) and Nieto & Sanchez-Navas (1994) suggest that the air-dried Ca-


10/14

124 L. N. Warror Sr-saturated illite 002 peak is a more suitablereflection for analysis, which is not interfered bythe presence of interlayered smecti te with twowater layers (d=15-) when expandibil i t ies areless that 20 % . A lthough not very common in verylow-grade metamorphic pelites, peak interferencewill occur in the presence of interlayered smectitecontaining one water layer (d= 12.5-). In the latter case, K-saturated samples may be used to induce collapse of the smectite layers to approximately 10- (Eberl et al., 1987).With respect to chlorite, the most commonlyused peak is the intense chlorite 002 air-dried reflection (e.g. Dandois, 1981; Braukmann, 1984;Arkai, 1991), which usually allows good fitting ofthe background level. However, this peak is strongly overlapped by the presence of kaolin (001 reflection), which is commonly encountered in verylow-grade metamorphic samples. For this reason,the chlorite 003 peak was preferred in this studyfor crystallite size calculations.Use of the illite 002 and chlorite 003 peaks(Fig. 2) gave the highest d egree of correlation (R 2= 0.95 5) in com pariso n to othe r basal illite andchlorite peak combinations (R 2 range of 0 . 8 1 1 -0.951), despite their significantly lower intensitiesin comparison with the more commonly used illite001 and chlorite 002 peaks. The illite 002/chlorite003 combination also showed an excellent correlation with the mineral facies assemblages. Thesetwo adjacent reflections can be accurately measured by an ultra-precise (very slow) high resolution scan of less than 6 20 , from which bo thpeaks can be simultaneously fitted. The proximityof these two reflections has the advantage that 20related differences are minimized, but the disadvantage that the peak-tails overlap on broad reflections.

Comparisons between FWHM and Fourierdetermined L data, calculated from illite 002 andchlorite 003 peaks, may also yield important information about the type of broadening, and hencethe nature of the X-ray scattering domains. This isillustrated in th e re gression data of F ig. 2 c, d,where FWHM data suggests a 1:1 relationship between illite and chlorite, in contrast to the S-Ldata which indicate that chlori tes are, on thewhole, slightly smaller in domain thickness thangrade equivalent i ll i tes. As FW HM measurem entsare less sensitive to broadening in the tails of theprofiles, than are the S-L and W-A methods, thesedifferences are probably reflective of more diffuseLorentzian type broadening in the limbs of the

chlorite peaks which may arise due to the presence of planar defects such as stacking faults (Er-gun, 1970; Moore & Reynolds, 1989). Careful examination of scaled and superimposed illite andchlorite peak profiles confirmed the presence ofslightly broadened chlorite tails in 9 of the 14samples. Another potential source of peak broadening is that of microstrain, but as no significantlevels of strain were recorded by either the W-A orS-L methods, this effect is largely discounted.

The W-A method (the preferred method ofEberl et al, 1988; Eberl & Blum, 1993; Warr &Rice, 1994), proved to be a slow, imprecise andinaccurate procedure for determining crystallitesize in polymineralic sam ples, probably a result ofits sensitivity to peak fitting errors and background corrections. This method is therefore onlyrecommended for more in depth study of mono-mineralic samples, when the crystallite size distribution is required, and where peak-fitting errorscan be minimized. The preferred method of thisstudy was the S-L Fourier method, which produced faster, more precise and apparently moreaccurate results than the W-A method. This is despite the reservations mentioned in the SiemensWIN-CRYSIZE handbook, which states that theS-L method should only be used when both crystallite size and microstrain effects are present.Preliminary constraints for the CrystallinityIndex Standard scaleCIS data from the very low-grade metamorphicrocks of southern New Zealand conform to a general pattern in which diagenetic grades correspondto the zeolite facies, anchizone mostly to the preh-nite-pumpellyite and pumpellyite-actinolite facies,and epizone to the greenschist facies (Fig. 2). Although there are too few samples to draw moreaccurate boundary limits, the greenschist to pum-pellyite-prehnite facies transition appears to liearound ICooi 0.2 5-0 .27 20, ChC 002 0 .24 -0 .31 20, while the boundary with the zeolite faciesoccurs in the range ICooi 0.3 2- 0. 51 20, CI1C0020.35 -0 .4 4 20 . These correlat ions correspondwell with the original anchizonal boundary limitsof Kiibler (IC 00 i 0.25 and 0.42 20) whichare adopted by the CIS scale. Employing the pe-trogenetic grid of Frey et al. (1991), and the correlations presented in Fig. 2, some general P-Tconstraints can be presented for the anchizonalrange of metamorphism presented in this study.Although the mineral facies overlap considerably


11/14

Clay mineral crystallinity and very low-grade metamorphism 12 5(Fig. 10 of Frey et al., 1991), correlating theanchizone with the prehni te-pumpel lyi te fac iesindica tes a P-T field from abou t 175 to 280 Cand 0.5 to 4.5 kbar, wh ile inclusion of the p um -pellyite-actinolite facies extends the temperaturefield up to a m aximu m of about 320 C at pressures below < 4 kbar.Further correlations for both crystallinity andcrystallite size values, based on regression analysis of the illite 001, 002 and chlorite 002, 003 reflections (Table 3) are preliminary, and intendedfor comparative purposes only. Correlation of thepeak breadths of illite 001 against chlorite 002show a proportional relationship, as recorded byDuba & Williams-Jones (1983), in contrast to thestudies of Dandois (1981), Braukman n (1984) andArkai (1991), which reported narrower chloritepeaks. Such differences may simply reflect varying amounts of mixed-layered smectite whichbroadens the illite 001 in respect to the chlorite002 peak, or alternatively such differences mayalso arise from variations in growth rates of therestrictive phyllosilicate minerals. A third alternative may b e related to the density of planar defectswithin the minerals . While correlations betweendifferent minerals do yield more crystal-chemicalinformation than considering one mineral alone(Arkai, 1991), and more mineralogical information is obtained by em ploying a wider range of basal reflections and measurement methods, cautionis required when comparing metamorphic boundary limits between different sample groups whichmay vary in both mineralogy and P-T-t history.

Conclus ions1. A pilot study of the clay mineral crystallinityand crystallite size of illites and chlorites from 14pel i t ic samples of known minera l fac ies conditions, suggest that the Crystallinity Index Standard (CIS) scale is a reliable indicator of metamorphic grade. However, before more accurate comparisons can be drawn between studies, adequateattention needs to be given to, A) calibration ofthe experimental data, and B) selection of suitablepeak reflections.

2. The illite 002 and chlorite 003 peaks provedthe two most useful peak reflections in this studyas they are broadened only by crystallite size effects, are minimally influenced by 20 dependentdifferences, and are less effected by other overlapping mineral phases.

3. The Siemens Single-Line method provideda more rapid and more precise measure of crystallite size than the Warren-Averbach method, as it isnot so sensitive to peak-fitting errors.4. The anchizonal range of illite and chloritecrystallinities (and crystallite sizes) from southernNew Zealand, measured by the CIS scale, corresponds to mineral facies with stability ranges between 175 to 320 C and 0.5 to 4.5 kbar. Differences between kinetically controlled clay mineralcrystallinity and thermodynamically based mineral facies assemblages are anticipated to exist between very low-grade metamorphic rocks of contrasting P-T-t histories.

Acknowledgements: Thanks to J . rodoh (Krakow) and an anonymous referee for reviewing thispaper, and to W. V. Maresch for his editorial comments. Also thanks to D. S. Coombs (Dunedin), S.C. Cox (Dunedin), M. Frey (Basel), H. J . Kisch(Negev), F. Nieto (Granada), A. H. N. Rice (Heidelberg), and W. B. Stern (Basel) for commentsand discussion on earlier versions of the manuscript. This paper is a contribution to IGCP 294.

ReferencesAprahamian, J. , Pair is, B., Pair is, J.L. (1975): Naturedes minraux argileux et cristallinit des illites dansle massif de Plate et le revers occidental desAiguilles Rouges: implications possibles d 'un pointde vue sdimentaire, structural , et mtamorphique.Ann. C entre. U niv. Savoie, 2, 9 5 - 1 1 9 .Arkai, P. (1983): Very low- and low-grade Alpine regional metamorphism of the Paleozoic and Mesozoicformations of the Biikkium NE-Hungary. Acta

Geol. Hung., 26, 8 3 - 1 0 1 .- (1991 ): C hlorite crystallinity: an emp irical approach and correlation with illite crystallinity, coalrank and mineral facies as exemplified by Paleozoicand M esozoic rocks of northwest Hungary. / . Metamorphic Geol., 9 , 7 2 3 - 7 3 4 .A rkai, P. & Torn, N .M . (1983): I l l ite crystall inity: com bined effects of domain size and lattice distortion.Acta Geol. Hung., 2 6 , 3 4 1 - 3 5 8 .Barker, C.E. & Goldstein, R.H. (1990): Fluid-inclusiontechnique for determining maximum temperature incalcite and its comparison to the vitrinite reflectance geothermometer. Geology, 18, 1003-1006.Bevins, R.E. & Robinson, D. (1993): Parageneses ofOrdovician sub-greenschist to greenschist faciesmetabasites from Wales, U.K. Eur. J. Mineral., 5,9 2 5 - 9 3 5 .Bishop, D.G. (1972): Progressive metamorphism fromprehnite-pumpellyite to greenschist facies in the


12/14

1 2 6 L . N . W a r rDansey Pass area, Otago, New Zealand. Bull. Geol.Soc.Amer., 8 3 , 3 1 7 7 - 3 1 9 8 .Braukmann, F.J. (1984): Hochdiagenese im Muschel-kalk der Massive von Bramsche und Vlotho.Bochumer geol. geotechn. Arb., 14, 195 pp.Bril , H. & Thiry, M. (197 6): L e mtamorphism e de bassepression anchi msozonal de la region deBodennec (Finistre): essai mthodologique. C. R.Acad. Sci. Paris. Sr. D, 283, 2 2 7 - 2 3 0 .Cashman, K.V. & Ferry, J .M. (1988): Crystal sizedistribution (CSD) in rocks and the kinetics anddynamics of crystallization III. Metamorphiccrystallization. Contrib. Mineral. Petrol., 9 9 , 4 0 1 -415.Coombs, D.S. & Cox, S.C. (1991) : Low- and verylow-grade metamorphism in southern New Zealand. Geol. Soc. N. Z. Misc. PubL, 58, 87 pp.C oombs , D .S . , Landi s, C. A ., N orr is, R .J . , Sinton, J .M.,Borns, D.J. , Craw, D. (1976): The Dun Mountainophiolite belt , New Zealand, i ts tectonic setting,constitution, and origin, with special reference tothe southern portion. Amer. J. Sci., 296, 5 6 1 - 6 0 3 .Dandois, Ph. (1981): Diagense et mtamorphisme dedoma ines caldonien et hercynien de la valle de laMeuse entre Charleville-Mzires et Namur (Ardennes franco-beiges). Bull. Soc. Belg. GoL, 90,2 9 9 - 3 1 6 .Duba, D. & Williams-Jones, A .E. (1983 ): The application of illite crystallinity, organic m atter reflectance,and isotopic techniques to mineral exploration; acase study in southwestern Gasp, Quebec. Econ.Geol., 78 , 1350-1363.Durney, D. (1974): Relations entre les temperaturesd'homognisation d'inclusions fluides et les min-raux mtamorphiques dans les nappes helvtiquesdu Valais. Bull. Soc. gol. Fr., 1 6 , 2 9 2 - 2 7 2 .Eberl, D.D. & Blum, A. (1993): I l l i te crystalli te thickness by X-ray diffraction. In Reynolds , R. C. &Walker, J. R . (eds.) , C ompu ter applications to X-raypowder diffraction, Clay Minerals Society workshop lec tures , 5 , 1 23 -15 3.Eberl, D.D. & rodori, J . (1988): Ostwald ripening andinterparticle-diffraction effects for illite crystals.Am. Mineral, 73, 1335-1345.Eberl, D.D., Srodori, J., Kralik, M., Taylor, B.E., Peter-man, Z.E. (1990): Ostwald ripening of clays andmetamorphic minerals. Science, 248, 4 7 4 - 4 7 7 .Eber l , D.D. , rodor i , J . , Lee , M., Nadeau, PH. , Nor throp, H.R. (1987): Sericite from the silvertonCaldera, Colorado: Correlation among structures,composition, origin, and particle thickness. Am.Mineral, 7 2 , 9 1 4 - 9 3 4 .Ehrenberg, S.N., Aagaard, P., Wilson, M.J., Fraser, A.R ., Duthie, D. M. L . (1993): Depth-dependent transformation of kaolinite to dickite in sandstones ofthe Norwegian continental shelf. Clay Minerals, 28,3 2 5 - 3 5 2 .Ergun, S. (1970): X-ray scattering by very defectivelattices. Phys. Rev. B, 1 , 3371-3380.

Fleming, C. A. (1970) : The Mesozoic of New Zealand;chapters on the history of the Circum-PacificMobile Belt . Q. J. geol Soc. London, 125, 1 2 5 - 1 2 7 .Frey, M., De Capitani, C , L iou, J .G . (1991): A newpetrogenetic grid for low-grade metabasites. / .Metamorphic Geol, 9 , 4 9 7 - 5 0 9 .Hunziker, J .C., Frey, M., Clauer, N., Dallmeyer, R. D.,Friedrichsen, H., Flehmig, W , H ochstrasser, K.,R oggwiler, P., Schwander, H. (1986): The evolutionof illite to muscovite: mineralogical and isotopicdata from the Glarus Alps, Switzerland. Contrib.Mineral Petrol, 92, 157-180.Jamieson, R . A . & C raw, D. (1987): Sphalerite geob aro-metry in metamorphic terranes; an appraisal withimplications for metamorphic pressure in the OtagoSchist. / . Metamorphic Geol, 8 , 1 3 - 3 0 .Kisch, H.J. (1981): Coal rank and ill i te crystallinityassociated with the zeolite fades of Southland andpumpellyite-bearing facies of Otago, southern NewZealand. N. Z. J. Geol. Geophys., 2 4 , 3 4 9 - 3 6 0 .- (198 7): C orrelation betwe en indicato rs of verylow-grade metamorphism. In Frey, M. (ed.) , LowTempera ture Metamorphism, 227-300; Blackie .(1990): Calibration of the anchizone: a criticalcomparison of illite "crystallinity" scales used fordefinition. /. Metamorphic Geol, 8 , 3 1 - 4 6 .Kisch, H.J. & Frey, M. (1987): Appendix: Effect ofsample preparation on the measured 10- peakwidth of illite (illite crystallinity). In Frey, M. (ed.),Low Tempera ture Metamorphism, 301-304;Blackie.Klug, H.R & Alexander, L.E. (1974): X-ray diffractionprocedures. 2nd edn., Wiley, New York, 966 p.Krumm, S. & Buggisch, W. (1991): Sample preparationeffects on illite crystallinity measurement: grain-size gradation and particle orientation. /. Metamorphic Geol, 9 , 6 7 1 - 6 7 7 .Krum m, S., Kisch, H.J. & Warr, L .N . (1994): Very-low-grade-Metamorphose - uneinheitliche Defini-tionen und Korrelationsmoglichkeiten - ErsteErgebnisse einer internationalen Laborvergleichs-untersuchung. Gottinger Arb. Geol Palont., 5,Sympos ium TSK, 76-78 .(in press): Interlaboratory study of the effects ofsample preparation on illite "crystallinity": aprogress report. Xlllth Conference on ClayMineralogy and Petrology, Prague.Kiibler, B. (1967): La cristallinit de i l l ite et les zonestout fait suprieures du mtamorphisme. In"Etages tectoniques Colloque de Neuchtel 1966",A la Baconnire, Neuchtel, Suisse, 105-121 .(1968): Evaluation quantitative du mtamorphismepar la cristallinit de i ll it e. Bull Centre Rech. PauSNPA, 2 , 3 8 5 - 3 9 7 .(1984): Les indicateurs de transformations physiques et chimiques dans la diagense, temperatureet calorimtrie. In Lagache, M. (ed.) , Thermomtrieet baromtrie gologiques, 489-596; Soc. fr .Mineral. Cristallogr., Paris.


13/14

Clay minera l c rys t a l l i n i ty and very low-grade metamorph i sm 127Kbler, B. (1990): "Cristallinit" de l 'illite et mixed-layers: brve revision. Schweiz. Mineral. Petrogr.Mitt., 70 , 8 9 - 9 3 .Kbler, B., Pittion, J.-L., Hroux, Y., Charollais, JWeidmann, M. (1979): Sur le pouvoir rflecteur dela vitrinite dans quelques roches du Jura, de laMolasse et des Nappes pralpines, helvtiques etpenniques. Ecologae geol. Helv., 7 2 , 3 4 3 - 3 7 3 .Liou, J.G., Maruyama, S. , Cho, M. (1985): Phase equilibria and mineral paragenesis or metabasites inlow-grade metamorphism. Miner. Mag., 49, 321333.MacKinnon, T.C. (1983): Origin of the Torlesse terraneand coeval rocks, South Island, New Zealand. Geol.Soc. Am. Bull., 9 4 , 9 6 7 - 9 8 5 .Merriman, R.J. , Roberts, B., Peacor, D.R. (1990): Atransmission electron microscope study of white

mica crystallite size distribution in a mudstone toslate transit ional sequence, N orth Wales, U .K .Contrib. Mineral. Petrol., 106, 2 7 - 4 0 .Moore, D.M. & Reynolds, R.C. (1989) : X-ray di f f raction and identification and analysis of clay minerals. Oxford University Press, 332 p.Nieto, F. & Sanchez-Navas, A. (1994): A comparativeXRD and TEM study of the physical meaning ofthe white mica "crystall inity" index. Eur. J.Mineral, 6, 6 1 1 - 6 2 1 .Norr is , R.J . & Bishop, D.G. (1990) : Deformedconglomerates and textural zones in the OtagoSchists, South Island, New Zealand. Tectono-physics, 174, 331 349 .Robinson, D. (1987): Transit ion from diagenesis tometam orphism in extensional and coll ision sett ings.Geology, 1 5 , 8 6 6 - 8 6 9 .Robinson, D. & Bevins, R.E. (1986): Incipient metamorphism in the Low er Paleozoic marginal basin ofWales. / . Metamorphic Geol, 4, 101-113.Sassi , F.P. & Scolari , A. (1974): The bo value of thepotassic white micas as a barometric indicator inlow-grade metamorphism of peli t ic schist . Contrib.Mineral. Petrol, 45 , 143-152 .

Siemens, WIN-CRYSIZE (1991): Crystall i te Size andMicrostrain (version 1.0) . Sigma-C GmbH, 59 p.rodori, J. (1984): X-ray diffraction of illitic minerals.Clays and Clay Minerals, 1 7, 2 3 - 3 9 .rodori, J. & Elsass, F. (1994): Effect of the shape offundamental particles on XRD characteristics ofillitic minerals. Eur. J. Mineral, 6, 113-122.Stern, W.B., Mullis, J. , Rahn, M., Frey, M. (1991):Deconvolution of the first "illite" basal reflection.Schw eiz. Mineral. Petrogr. Mitt., 7 1 , 4 5 3 - 4 6 2 .Warr, L.N. (1993): A calibration approach to the standardization of XRD clay mineral crystallinity andcrystallite (domain) size data. Terra A bstracts, 5,421 .Warr, L .N . & R ice, A .H .N . (1994) : In ter laboratorystandardization and calibration of clay mineralcrystallinity and crystallite size data. J. Metamorphic Geol, 12, 141-152.Warren, B.E. (1959): X-ray studies of deformed metals.Prog. Met. Phys., 8 , 147-202 .Warren, B.E. & Averbach, B.L. (1950): The effect ofcold-work distortion on X-ray patterns. /. Appl.Phys., 2 1 , 5 9 5 - 5 9 9 .Weber, R, Dunoyer de Segonzac, G., Economou, C.(1976): Une nouvelle expression de la "cristallinit" de i l l ite et des micas. Notion d 'paisseurapparente" de cristall i tes. C.R. somm. Soc. gol.Fr., 5 , 2 2 5 - 2 2 7 .Wilson, A .J .C . (1963): Mathem atical theory of X-raypowder diffractometry. Ed. Philips TechnicalLibrary, Eindhoven.Yardley, B . W D . (1982) : The ear ly metamorphic his toryof the Haast Schists and related rocks of NewZealand. Contrib. Minera l Petrol, 8 1 , 3 1 7 - 3 2 7 .

Received 20 September 1994Modified version received 25 July 1995Accepted 13 September 1995


14/14

warr 1996 standardised clay crystall data low grade nz

Documents